Guest Post: Jerry and Julie Jive

“Good news, everyone!” to quote Prof. Farnsworth. Our good friends Jerry DeSilva and Julie Lesnik just published a paper in the Journal of Human Evolution, about neonatal brain size in primates [1]. Rather than talk and talk about it, probably missing the important stuff, I made some calls. The authors were kind enough to make a cameo appearance at Lawn Chair to talk about their paper about their paper. Thanks, Jerry and Julie! Here’s what they had to say:

Summary:

This paper presents a regression equation that can be used to calculate the size of the brain at birth in different hominin species.

Significance:

Knowing the size of the brain at birth is critical for understanding obstetric constraints and brain development throughout human evolution. Unfortunately, it is very unlikely to find fossil evidence of how big the brain was at birth in human ancestors (though see below). This paper presents a way to get around the absence of fossil evidence and calculate the size of the neonatal brain in early homs using what we know about brain development in modern primates.

Things Jerry liked about the paper:

Humans are so unusual, and in biological anthropology we often study ways in which humans are different from other primates. However, what this study finds is that humans are no different from other primates in terms of the adult/neonatal brain scaling relationship. This means that we have exactly the brain size at birth expected given the size of our brains as adults. Because of this, we can infer that our extinct ancestors and relatives also followed this ‘rule’ of adult/neonatal brain size, and can calculate the size of the brain at birth from reliable estimates of brain size in 89 adult fossil crania that have been unearthed.

I am also thrilled that Julie and I may have solved the “% brain size at birth” issue that has been all over the literature lately. Did Homo erectus have a more human-like or a more chimpanzee-like pattern of brain growth? What about australopiths? Well, we’ve found that the whole issue of % brain size at birth is simply a function of the scaling relationship between adult and neonatal brain size. Because they do not scale 1:1, but instead scale 1:0.73 (roughly), as the adult brain gets bigger, the neonatal brain gets proportionately smaller. Therefore, less of brain growth occurs in the womb as overall adult brain size increases. If you know the size of a hominin brain as an adult (which we do from the many preserved fossil crania), you can calculate the size of the brain as a baby, and then easily take a % of how much of that brain growth is achieved by birth.

Again, because of the negative allometry (m=0.73), we argue that % of brain size at birth in hominins was never “chimpanzee-like” or “human-like”, but instead followed a gradual progression from a chimpanzee-like ancestral condition to what we have today.

Things Julie liked about the paper:

So much is going on when we think about hominid evolution, especially in the early Pleistocene.With the emergence of Homo brain size is increasing, bipedality is becoming more efficient, and tool use is becoming more advanced.What I like about this paper is that understanding neonatal brain size is one way of tying all of those elements together.Humans are considered to be secondarily altricial meaning that they are born in a more underdeveloped state than their ancestors.Selection for this smaller neonatal size is often considered to be linked to the constraints placed on the pelvis by selection for more efficient bipedal locomotion.A small brain size at birth and a large adult brain always seemed exceptional for Homo.What our paper shows is that the relationship is entirely normal across anthropoids.So, where is the selective pressure?On the larger brain as an adult or on the smaller brain as a newborn?I am now more apt to lean towards larger adult brain.Efficient bipedality is important for exactly that reason; it’s efficient and therefore requires less energy to walk upright and allows the body to allot that energy to other tasks, such as maintenance of a large brain.Add tool-use advancement to the equation and it seems bigger brains and more advanced cognitive abilities were of primary importance at this stage of human evolution.

What we’d do different:

I would have included Neandertals. Julie and I made a statement in the introduction that the discovery of neonatal crania was bordering on impossible. Just days before our paper appeared on-line, however, Marcia Ponce de Leon published a fantastic paper in PNAS on a neonatal Neandertal cranium from Mezmaiskaya Cave in Russia [2]. What is very exciting to me is that this newly described fossil allows us to test our regression equation. How accurate is it in predicting the size of the brain at birth in Neandertals (which we now know because of this new specimen)? Our regression would predict a brain size of about 425 cc, which is very close to the size of the brain at birth in the Mezmaiskaya infant and well within the 95% CI. When two independent methods arrive at the same result, it is reasonable to argue that the method is valid.